The rapid development of optoelectronics has produced a need for methods of producing nanometer-sized patterns on semiconductor surfaces. Typically, these patterns are etched into semiconductor substrates by a number of techniques. For example, wet chemical etching has been used to etch a wide variety of semiconductors. However, wet chemical etching can be isotropic, thereby limiting the aspect ratio of features that can be fabricated, and the uniformity of the process is low. Production industries in the optoelectronics industries require more controlled etching procedures than can be achieved using wet etching.
As a result of the shortcomings of wet chemical etching, dry etching processes have been developed for semiconductors. For example, reactive ion etching has been used to produce well-controlled etching profiles in silicon substrates. Reactive ion etching involves generating chemically reactive species, such as radicals and ions, via an electric discharge in a low pressure reactive gas mixture in a reactor. The reactive species generated in this manner are accelerated towards a substrate by means of an electrical field and react with the silicon to produce volatile reaction product which are pumped away. An etch mask can be applied to the substrate prior to etching to allow the reactive species to etch a pattern into the substrate. Because of the nearly vertical fall of the positively-charged reactive species, etching is much slower on the side-walls of the etched features as the etching progresses into the substrate.
Hard etch masks that are made of, for example, SiO2, Si3N4, or metals are well known to those of ordinary skill in the art of semiconductor photolithography. But these masks are difficult to fabricate since they typically need to be vacuum deposited in a separate step before the photoresist is applied. Hard etch masks make the process more complicated and add more process steps. Additionally, after etching the hard masks need to be removed by dry etching or wet etching. Furthermore, with many etching chemistries, these materials tend to etch at a rate that is close to that of many II-VI or III-V semiconductors which makes their use limited. While reactive-ion etching is very useful for semiconductors, such as silicon, that form volatile byproducts which can be easily eliminated from a vacuum chamber via pumping, reactive-ion etching is not very practical for II-VI semiconductors since these materials do not easily react with reactive-ions and typically do not form volatile byproducts. Dry etching is well-established for patterning most semiconductor materials. Chlorine (Cl2) based reactive ion etching (RIE) is widely used in dry etching of III-V and II-VI semiconductors for fabricating various optoelectronic devices and detectors. Other gas systems, including Cl2/Ar, Cl2/N2, Cl2/He, Cl2/BCl3/Ar, BCl3/Ar, BrCl3, SiCl4/Ar, CCl2F2/H2/Ar, etc., have also been investigated. However, these reactants and their products are known to be corrosive and toxic. Also, rapid post-etching degradation has been observed when Cl-based RIE is used, due to the corrosion by persistent Cl2 residues. In the specific case of cadmium-containing semiconductors, cadmium halides have vapor pressures which are several orders of magnitude too low to provide the basis for useful etching. In the absence of cadmium volatility, the most likely result is the formation of a cadmium-rich material on the surface of the semiconductor. Therefore, CH4/H2 and CH4/H2/Ar based chemistries have been developed and are favored for plasma etching of Cd-containing semiconductors. However, these processes suffer from several drawbacks including extensive polymer deposition, which acts as an etch stop mechanism, rough surfaces, and low etch rates (less than 50 nm/min).